As one of the most preferred types of resonators for microwave devices, various cavity resonators have been realized in microwave packaging structures. If such cavity resonators are employed within packages, especially for filters, it is critical to achieve a sufficient coupling level from a feedline to the resonator since the amount of the achievable coupling defines the range of the bandwidth for which a microwave device can be designed. However, the design rules used for manufacturing often prevent structures from realizing the desired amount of coupling.
There are cavity resonators known, for instance from L. Harle et al. “The effects of slot positioning on the bandwidth of a micromachined resonator”, in proceeding of 28th European Microwave Conference, 1998, pp. 664-668. Here, the cavity is fed by using slot coupling with a planar transmission line (feedline), such as a microstrip line. However, such a coupling is too weak and with this type of coupling, the bandwidth of a microwave device becomes too narrow for many applications.
To increase the amount of the coupling, other types of feeding structure for cavity resonators have been developed, e.g. from Lee et al. “Comparative study of feeding techniques for three-dimensional cavity resonators at 60 GHz”, IEEE Transactions on Advanced Packaging, Vol. 30, No. 1, Feb. 2007, pp. 115-123. In such cavity resonators, for coupling between a cavity and its planar feedline a via probe is provided that reaches into the cavity with a gap from the bottom of the cavity since coupling from the slot and feedline is often too weak to obtain a critical coupling level for filter applications. However, a precise manufacturing of the via probe is ultimately required with additional layer masks to implement the gap.
When referring hereinafter to microwave frequencies, a frequency range from at least 0.3 GHz to 3 THz shall be generally understood, i.e. including frequencies commonly referred to as millimeter-wave frequencies.